AFM is a condition that affects the nervous system and causes muscle weakness, according to the Center for Disease Control and Prevention (CDC). In particular, the condition can cause weakness in the arms and legs along with loss of muscle tone and problems with reflexes. Other symptoms include facial drooping, difficulty moving the eyes, difficulty swallowing and slurred speech, the CDC says. Most cases are in children.

The condition is not new, but officials started to see a rise in cases in 2014. Since then, more than 350 cases of the illness have been reported in the U.S. over a four-year period. So far this year, there have been 38 cases in 16 states, the CDC says.

The cause of AFM, and the reason for the rise in cases starting in 2014, is not known. However, the 2014 cases coincided with a national outbreak of a respiratory illness caused by a virus called enterovirus D68. It's possible that AFM has a variety of causes, including viruses (such as poliovirus and enteroviruses), environmental toxins and genetic disorders, the CDC says.